This invention relates to wastewater treatment and more particularly, to the treatment of undigested wastewater sludges. The invention includes both an apparatus and method for economically oxidizing sludges to produce products which may be discharged into the environment or readily disposed of.
Municipal and other wastewater treatment plants use various processes to break down the raw sewage influent and produce a sewage sludge. The final sludge product includes water, organic material, and smaller quantities of inorganic and inert material.
A typical wastewater treatment plant first produces a primary sludge collected from a primary clarifier or sedimentation unit. A secondary sludge is collected from a holding basin after a process is applied to the material remaining after removal of the primary sludge. The water separated from the secondary sludge is commonly treated with a disinfecting agent such as chlorine and then discharged from the plant. The process used to produce the secondary sludge may be a biological process such as an activated sludge process, a trickling filter system, an aerated lagoon, or a rotating biological contactor. The process may also be a physical-chemical process. The combined primary and secondary sludges are commonly thickened, and then digested to further break down the organic material. Finally, the digested sludge is dewatered to produce a material which may be disposed of in some manner.
There are a number of problems associated with these common wastewater treatment plants. First, the facilities are expensive and difficult to operate and generally provide no useful products. The sludge from some wastewater treatment facilities may be composted to produce a material suitable for use as a soil amendment or fertilizer, however, this by-product is made only at considerable cost. Sludges which are not used to make a compost leave the problem of disposal.
A hydrothermal process known as supercritical water oxidation has been suggested for completely oxidizing digested sewage sludges and other organic wastes. Another hydrothermal process commonly referred to as wet air oxidation has been used for oxidizing various organic materials. As used herein, "wet air oxidation" refers to a hydrothermal oxidation process conducted at lower than the critical temperature for water, whereas "supercritical water oxidation" refers to a hydrothermal oxidation process which is carried out at supercritical conditions for water, that is, at or above the critical temperature and the critical pressure. The critical temperature for pure water is approximately 705.degree. F. (374.degree. C.) while the critical pressure is 3199 psia (220.4 bar). The goal of either process is to destroy the organic material in the sludge by oxidation. While wet air oxidation generally cannot achieve complete oxidation of a particular feed, supercritical water oxidation may oxidize substantially all organic material in the reaction mixture leaving water, CO.sub.2, N.sub.2, and inorganic materials such as metals, salts, sand, and clay.
The amount of organic material to be destroyed in a waste such as a sewage sludge may be described in terms of the chemical oxygen demand or COD of the material. Generally, the COD of a given material is the amount of oxygen required to completely oxidize the material. Also, both wet air oxidation and supercritical water oxidation are exothermal reactions and the feed materials for the reactions may be described in terms of their heating value, commonly expressed in Btu/pound of feed material. Sewage sludges may further be described in terms of volatile suspended solids or VSS in weight percent. VSS may be defined as the relative organic material content of the total mixture.
U.S. Pat. No. 4,338,199 to Modell (the "'199 Patent") suggests that sewage sludges may be reacted with an oxidant at temperatures and pressures at supercritical conditions to substantially remove all COD from the sludge. The '199 Patent teaches initiating the oxidation reaction only at supercritical conditions to achieve the desired destruction of organic material. The disclosed system requires a feed having a low organic material concentration or COD in order to maintain the reaction temperature at acceptable levels in the supercritical water oxidation reactor. Temperatures above approximately 1100.degree. F. to 1200.degree. F. may weaken reactor vessel materials to a point at which the material is unable to withstand the force from the pressure of the reaction mixture. Furthermore, reaction temperatures above 1400.degree. F. may cause the formation of NO.sub.x.
U.S. Pat. No. 5,240,619 to Copa (the "'619 Patent") discloses a two stage oxidation reaction for high strength wastewaters. In the process shown in the '619 Patent, most of the COD is removed in a wet air oxidation reaction conducted in a separate reactor. Any remaining COD is removed in a supercritical water oxidation reactor.
Neither the '199 Patent, nor the '619 Patent is directed to the treatment of undigested sewage sludges. The '199 Patent cannot accommodate the oxidation of high strength undigested sewage sludges and relies on digestion of the sludge or dilution to produce a feed mixture having a lower COD and heating value. The system disclosed in the '619 Patent requires a separate subcritical or wet air oxidation reactor to retain the material at subcritical conditions for a sufficient time to remove the bulk of the COD. Thus, the '619 Patent process requires additional equipment and relatively long residence times to accommodate the relatively slow separate subcritical oxidation process.
U.S. Pat. No. 5,433,868 to Fassbender (the "'868 Patent") is directed specifically to the treatment of dewatered sewage sludge and primarily to the problem of removing nitrogen compounds from the discharge stream of the treatment plant. The process disclosed in the '868 Patent preferably includes liquefying the sewage sludge in an alkaline digestion process to produce a low ammonia content stream and a high ammonia content stream. This high ammonia content stream is subjected to a hydrothermal process to destroy most of the ammonia in the stream. In another embodiment, the '868 Patent suggests that an undigested, dewatered sewage sludge may be treated with a hydrothermal process to remove nitrogen. However, where the hydrothermal process is applied to the complete dewatered undigested sludge, the removal efficiency for nitrogen compounds is reduced. In any event, the effluent from the hydrothermal process is not a clean product and must be returned to the treatment plant ahead of the primary clarifier.